The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
There is a continuing need for higher performance digital data communications systems. Perhaps nowhere is this need more evident than on the worldwide packet data communications network now commonly referred to as the “Internet.” On the Internet, the “richness” of content is constantly increasing, requiring an ever-increasing amount of bandwidth to provide Internet content to users. As a result of this increased demand for bandwidth, significant efforts have been made to develop new types of high-speed digital data communications systems. For example, optical fiber based networks are being built in many large metropolitan areas and undersea to connect continents. As another example, new wireless protocols are being developed to provide Internet content to many different types of small, portable devices.
One of the significant drawbacks of deploying many of these new types of high-speed digital data communications systems is the high cost and amount of time required to develop and build out the new infrastructure required by the systems. Because of these high costs, many new high-speed digital data communications systems are initially deployed only in densely populated areas, where the cost of building out the new infrastructure can be quickly recovered. Less populated areas must often wait to receive the new communications systems and some rural areas never receive the new systems where it is not cost effective to build the infrastructure.
For several reasons, significant efforts are being made to utilize conventional twisted pair telephone lines to provide high-speed digital data transmission. First, a significant amount of twisted pair telephone line infrastructure already exists in many countries. Thus, using conventional twisted pair telephone lines avoids the cost of building expensive new infrastructure. Second, conventional twisted pair telephone lines extend into customers' homes and businesses, avoiding the so-called “last mile” problem. As a result of recent development efforts in this area, several new communications protocols, such as Asymmetric Digital Subscriber Line (ADSL), G.Lite and Very High Bit Rate DSL (VDSL), have been developed for providing high-speed digital transmission over conventional twisted pair telephone lines.
Despite the advantages to using conventional twisted pair telephone lines to provide high-speed digital communications, there are some problems with this approach. First, conventional twisted pair telephone lines cause signal attenuation per unit length that increases rapidly with frequency. A moderate length twisted pair line, for example around fifteen thousand feet, may cause only a few decibels (dB) of attenuation in the voice band, for which the line was originally designed, but many tens of dB of attenuation at higher transmission frequencies, for example around 1.1 MHz for ADSL. This results in a transfer function with a wide dynamic range and a wide variation in group-delay, making communications channel equalization more difficult. The transfer function is further complicated by bridge taps and impedance mismatches between line sections that cause reflections and echoes at the receiver. Furthermore, filtering performed at the transmitter and receiver also increases the complexity of the transfer function. The result is a very long communications channel impulse response, which creates significant inter-symbol interference (ISI) in a conventional digital communications receiver due to the resulting overlap of adjacent symbols and also may result in inter-channel interference (ICI).
The standards for ADSL and G.Lite specify Discrete Multitone (DMT) modulation. DMT is also under consideration for use in VDSL systems. DMT modulation generally involves transmitting digital data on a number of carriers simultaneously. Modulation and demodulation are performed using a Fast Fourier Transform (FFT). A cyclic prefix is added to the data prior to transmission to ensure separation between successive DMT symbols and eliminate inter-symbol interference (ISI) and to help reduce inter-channel interference (ICI) at the receiver. In practice, the cyclic prefix is necessarily quite short, generally much shorter than the impulse response of the communications channel. This often results in significant ISI and ICI being present in the received data. Large amounts of ISI and ICI cause a large reduction in the available communications bandwidth. This is especially true for long twisted pair telephone lines likely to be encountered in ADSL and VDSL communications systems. The effect of this ISI is to reduce the signal to noise ratio (SNR) in each bin of the FFT demodulator employed in a DMT system.
Standard equalizers used in digital communications systems, such as adaptive least mean square (LMS), Kalman, and recursive least squares (RLS) equalizers, are generally inappropriate for DMT systems since such standard equalizers are not designed to eliminate ISI and ICI. If the impulse response of the overall communications channel and equalizer is longer than the cyclic prefix, ISI and ICI can still occur. As a result, conventional approaches for equalizer design have the objective of shortening the overall communications channel plus equalizer impulse response so that the overall impulse response is shorter than the length of the cyclic prefix. Various attempts to reduce the overall impulse response to less than the length of the cyclic prefix have been made, including the use of finite impulse response (FIR) equalizers. See for example, Impulse Response Shortening for Discrete Multitone Transceivers, by P. Melsa, R. Younce, and C. Rohrs, IEEE Transactions on Communications, Vol. 44 No. 12, December 1996; Optimum Finite-Length Equalization for Multicarrier Transceivers, by N. Al-Dhahir and J. Cioffi, IEEE Transactions on Communications, Vol. 44 No. 1, January 1996; and A Discrete Multitone Transceiver System for HDSL Applications, by J. Chow, J. Tu, and J. Cioffi, IEEE Journal on Selected Areas in Communications, Vol. 9 No. 6, August 1991. Determining equalizer coefficients is generally a computationally inefficient process and can be quite sensitive to noise, which limits the practical application of the techniques found in these references.
A problem with conventional approaches that shorten the overall impulse response is that no consideration is given to the spectrum of noise on the communications channel. In particular, a common misconception of the conventional approaches is that a FIR equalizer leaves the signal to noise ratio (SNR) unaffected if the FIR equalizer eliminates ISI by shortening the impulse response. This misconception arises from an inadequate analysis of how demodulation by a non-windowed finite length FFT affects the noise power on the demodulated tones. As used herein, the term “tones” is synonymous with “frequencies.”
The effect of the non-windowed FFT is to cause a smearing of the equalized noise power across the tones on the communications channel. The smearing affect can be very pronounced for narrow band interference sources, such as amplitude modulation (AM) radio signals. In addition, deep nulls in the equalize frequency response cause an attenuation of the signal, which is not reproduced in the noise, and as a result, even white noise at the input can be spread across the tones. The result of the smearing of the noise is a reduction in the SNR on the affected tones. Due to this reduction in the SNR, conventional approaches attempt to limit the range of the equalizer frequency response, but doing so in an ad hoc manner may result in an unacceptably large ISI.
Based on the foregoing, there is a need for an approach for processing data received from a communications channel that takes into account noise while reducing inter-symbol interference and inter-channel interference and that does not suffer from the limitations of conventional approaches.